For this type of buried-ridge multi-section electro-optical component, it is important to provide a high degree of electrical isolation between each section. Typically, it is necessary to provide resistance of not less than 15 k.OMEGA. at the interface between two sections, so as to prevent the sections from interacting with each other while the component is operating.
By way of example, FIG. 1 is a diagram in longitudinal section through an in-line transmitter-receiver device (1D-TRD) obtained by monolithically integrating a laser 30 and a detector 20 on the same substrate 10. The laser 30 transmits a signal, e.g. at a wavelength of 1.3 .mu.m to an optical fiber 50, and the detector receives a signal from the same optical fiber 50, e.g. at a wavelength of 1.55 .mu.m. In this example, since the transmit wavelength is shorter than the receive wavelength, the component includes a third section, disposed between the laser 30 and the detector 20, and forming an optical isolator 40. The purpose of the optical isolator 40 is to absorb the light emitted, at 1.3 .mu.m, via the rear face of the laser, and thereby to prevent the detector from being dazzled, so that said detector can detect the optical signal at 1.55 .mu.m coming from the optical fiber. The substrate 10 may, for example, be made of n-doped InP. The waveguides (21, 31) respectively of the detector 20, of the laser 30, and of the optical isolator 40 are etched in the form of ridges and buried in a layer of highly-doped optical cladding 11. The waveguides may be said to be of the "Buried Ridge Structure (BRS)" type. For example, the cladding material is p+ doped when the substrate is n-doped. Since said material is highly doped, it is made conductive.
FIG. 2A is a plan view of the component shown in FIG. 1, and FIG. 2B is a view in cross-section on A--A. In FIG. 2B, conductive channels situated on either side of the waveguide 31 are referenced 12. The presence of the conductive channels is due to the fact that the cladding layer 11 is highly doped. Proton implantation or lateral etching, referenced 15 in the diagram of FIG. 2B, and making the material resistive, also enables the width of the conductive channels 12 to be limited to a value typically lying in the range 10 .mu.m to 12 .mu.m, and enables the component to be isolated laterally. However, the conductive channels 12 prevent very high resistance from being obtained at the interface between two sections.
The compositions and dimensions chosen for the waveguides are of little importance. In the example shown in FIG. 1, the waveguide 21 of the detector is, for example, made of a ternary material, while the waveguide 31 of the laser and of the optical isolator 40 is in the form of a quantum-well structure.
Furthermore, metal electrodes 22, 32, 42, 13 are formed on the various sections and on the bottom face of the component so as to enable it to operate.
Because of the presence of the conductive channels 12 on either side of the waveguide ridges, it is necessary to form electrical isolation zones or "resistive zones" I, between the various sections 20, 30, 40 in order to prevent any section from interfering with another while the component is operating.
In order to form the resistive zones I, and in order to obtain very high interface resistance between two consecutive sections, three solutions have been studied so far.
The first solution consists in growing, by epitaxy, a semi-insulating indium phosphide material (InP:Fe) at the interfaces between the sections. Unfortunately, that solution cannot yet be adapted to satisfy the constraints of industrial production. In addition, it requires an additional growth step, which is unacceptable if a low-cost component is to be obtained.
The second solution consists in etching the cladding material 11 in the zones I situated at the interfaces between the sections.
The third solution consists in implanting protons in the cladding material 11 locally in the zones I situated at the interfaces between the sections. In which case, the protons generate crystal defects which enable the population type to be reversed. Thus, the cladding material 11 is made locally semi-insulating, and therefore highly resistive.
Unfortunately, the second and third solutions suffer from a major drawback. In order to obtain sufficient electrical isolation, i.e. in order to obtain interface resistance of not less than 15 k.OMEGA., it is necessary to perform the etching or the proton implantation to a great depth, and in particular through the waveguides (see FIG. 1). Etching a waveguide or implanting protons in the core of a waveguide gives rise to crystal defects in the material. Such crystal defects generate light-diffusing centers, thereby giving rise to optical losses at the interfaces between the sections. Operation of the component is degraded by the optical losses. In addition, the higher the number of interfaces, the higher such optical losses become. The light signal, at 1.55 .mu.m, which is output by the optical fiber 50 and injected into the detector 20 has to pass through two interfaces consecutively. The optical losses are therefore very high.
By way of example, for a received signal, at a wavelength of 1.55 .mu.m, the optical losses, due to proton implantation at a density of 2.times.10.sup.14 cm.sup.-2, are about 0.8 dB for an implanted distance of length 5 .mu.m, which corresponds to losses of about 3 dB for a component having two implanted sections of length 10 .mu.m.
In addition, curve a shown in FIG. 3 represents the value of the interface resistance R as a function of the distance h between the bottom of the implantation zone (or of the etching zone) I and the top surface of the waveguide, for a component shown in cross-section in FIG. 3a and similar to the buried active ridge component shown in FIG. 1. The curve shows that, in order to achieve resistance of not less than 15 k.OMEGA., it is necessary to implant protons through the waveguide (or to etch through the waveguide), so that the bottom of the resistive zone I is situated at a distance h approximately in the range 0.8 .mu.m to 0.9 .mu.m below the top surfaces of the waveguides.
Curve b in FIG. 3 shows the value of the interface resistance R as a function of the distance h between the bottom of the implantation zone (or of the etching zone) and the top surface of the waveguide for a prior art component in which the waveguide is of the ridge type, i.e. it is not etched. That component is shown in cross-section in FIG. 3b, and it includes a top layer 14 which is deposited on the waveguide and which is etched over a width and a length that are determined to create a refractive index contrast, and to make it possible to guide light. In that component, the waveguides are not buried. Satisfactory resistance values R are obtained for implantation (or etching) depths in the top layer 14 that are sufficiently shallow, i.e. for depths such that the bottom of the resistive zone I is situated at a distance h lying in the range 0.2 .mu.m to 0.1 .mu.m above the top surface of the waveguide. Therefore, in that case, the waveguide is not degraded by the implantation (or the etching).